The present invention relates to electrostatic inkjet print technologies and, more particularly, to printheads and printers of the type such as described in WO 93/11866 and related patent specifications.
Electrostatic printers of this type eject charged solid particles dispersed in a chemically inert, insulating carrier fluid by using an applied electric field to first concentrate and then eject the solid particles. Concentration occurs because the applied electric field causes electrophoresis and the charged particles move in the electric field towards the substrate until they encounter the surface of the ink. Ejection occurs when the applied electric field creates an electrophoretic force that is large enough to overcome the surface tension. The electric field is generated by creating a potential difference between the ejection location and the substrate; this is achieved by applying voltages to electrodes at and/or surrounding the ejection location. One particular advantage of this type of print technology is the ability to print using greyscale, using control of the applied voltages to modulate the volume of ink ejected.
The location from which ejection occurs is determined by the printhead geometry and the position and shape of the electrodes that create the electric field. Typically, a printhead consists of one or more protrusions from the body of the printhead and these protrusions (also known as ejection upstands) have electrodes on their surface. The polarity of the bias applied to the electrodes is the same as the polarity of the charged particle so that the direction of the electrophoretic force is towards the substrate. Further, the overall geometry of the printhead structure and the position of the electrodes are designed such that concentration and then ejection occurs at a highly localised region around the tip of the protrusions.
To operate reliably, the ink is arranged to flow past the ejection location continuously in order to replenish the particles that have been ejected. To enable this flow the ink must be of a low viscosity, typically a few centipoise. The material that is ejected is more viscous because of the concentration of particles; as a result, the technology can be used to print onto non-absorbing substrates because the material will not spread significantly upon impact.
Various printhead designs have been described in the prior art, such as those in WO 93/11866, WO 97/27058, WO 97/27056, WO 98/32609, WO 01/30576 and WO 03/101741, all of which relate to the so-called Tonejet® method described in WO 93/11866.
FIG. 1 is a drawing of the tip region of an electrostatic printhead 1 of the type described in this prior art, showing several ejection upstands 2 each with a tip 21. Between each two ejection upstands is a wall 3, also called a cheek, which defines the boundary of each ejection cell 5. In each cell, ink flows in the two pathways 4, one on each side of the ejection upstand 2 and in use the ink meniscus is pinned between the top of the cheeks and the top of the ejection upstand. In this geometry the positive direction of the z-axis is defined as pointing from the substrate towards the printhead, the x-axis points along the line of the tips of the ejection upstands and the y-axis is perpendicular to these.
FIG. 2 is a schematic diagram in the x-z plane of a single ejection cell 5 in the same printhead 1, looking along the y-axis taking a slice through the middle of the tips of the upstands 2. This figure shows the cheeks 3, the ejection upstand 2, which defines the position of the ejection location 6, the ink pathways 4, the location of the ejection electrodes 7 and the position of the ink meniscus 8. The solid arrow 9 shows the ejection direction and also points towards the substrate. Each upstand 2 and its associated electrodes and ink pathways effectively forms an ejection channel. Typically, the pitch between the ejection channels is 168 μm (this provides a print density of 150 dpi). In the example shown in FIG. 2 the ink usually flows into the page, away from the reader.
FIG. 3 is a schematic diagram of the same printhead 1 in the y-z plane showing a side-on view of an ejection upstand along the x-axis. This figure shows the ejection upstand 2, the location of the electrode 7 on the upstand and a component known as an intermediate electrode (10). The intermediate electrode 10 is a structure that has electrodes 101, on its inner face (and sometimes over its entire surface), that in use are biased to a different potential from that of the ejection electrodes 7 on the ejection upstands 2. The intermediate electrode 10 may be patterned so that each ejection upstand 2 has an electrode facing it that can be individually addressed, or it can be uniformly metallised such that the whole surface of the intermediate electrode 10 is held at a constant bias. The intermediate electrode 10 acts as an electrostatic shield by screening the ejection channel from external electric fields and allows the electric field at the ejection location 6 to be carefully controlled.
The solid arrow 11 shows the ejection direction and again points in the direction of the substrate. In FIG. 3 the ink usually flows from left to right.
In operation, it is usual to hold the substrate at ground (0 V), and apply a voltage, VIE, between the intermediate electrode 10 and the substrate. A further potential difference of VB is applied between the intermediate electrode 10 and the electrodes 7 on the ejection upstand 2 and the cheeks 3, such that the potential of these electrodes is VIE+VB. The magnitude of VB is chosen such that an electric field is generated at the ejection location 6 that concentrates the particles, but does not eject the particles. Ejection spontaneously occurs at applied biases of VB above a certain threshold voltage, VS, corresponding to the electric field strength at which the electrophoretic force on the particles exactly balances the surface tension of the ink. It is therefore always the case that VB is selected to be less than VS. Upon application of VB, the ink meniscus moves forwards to cover more of the ejection upstand 2. To eject the concentrated particles, a further voltage pulse of amplitude VP is applied to the ejection upstand 2, such that the potential difference between the ejection upstand 2 and the intermediate electrode 10 is VB+VP. Ejection will continue for the duration of the voltage pulse. Typical values for these biases are VIE=500 volts, VB=1000 V and VP=300 volts.
The voltages actually applied in use may be derived from the bit values of the individual pixels of a bit-mapped image to be printed. The bit-mapped image is created or processed using conventional design graphics software such as Adobe Photoshop and saved to memory from where the data can be output by a number of methods (parallel port, USB port, purpose-made data transfer hardware) to the printhead drive electronics, where the voltage pulses which are applied to the ejection electrodes of the printhead are generated.
One of the advantages of electrostatic printers of this type is that greyscale printing can be achieved by modulating either the duration or the amplitude of the voltage pulse. The voltage pulses may be generated such that the amplitude of individual pulses are derived from the bitmap data, or such that the pulse duration is derived from the bitmap data, or using a combination of both techniques.
To obtain high quality inkjet printing, the ejection process must be identical on each occasion, so that jet volume, jet composition, etc., are consistent. Inkjet printing is a dynamic process and the various physical processes involved each occur on characteristic timescales.
However, in certain circumstances, ejection could be delayed from the start of the voltage pulse as a result of the timescale at which ejection events may occur. The manner in which a print channel starts ejecting depending on how long it has been dormant is usually referred to its “start-up” characteristic; in the special case of a single dot or pixel this characteristic is called “drop-on-demand”.
In a similar way to that in which a delay in start-up of ejection might occur at the beginning of printing after a pause or block of non-printed pixels, so a delay in turn-off could exist. The result, continued ejection after the end of a block or line of pixels, might result in uneven and lengthened ends of printed areas.
The dynamic processes involved during electrostatic printing include electrostatic and fluidic effects. Two of these dynamic processes are:                1. During operation, but when ejection is not required, a d.c. bias voltage, VB, is applied to the ejection location and the ink meniscus relaxes into a stable shape, pinned to the ejection location and other features on the printhead. When a voltage pulse is applied (VB+VP), the meniscus moves forwards and ejection occurs in the manner described earlier. Once the voltage pulse has finished, the ink will relax back to its initial state, but if a subsequent pulse is applied before this relaxation has finished, the resultant ejection event will start with the meniscus in a forward and therefore more suitable position. The resultant print density will therefore be stronger than for the first ejection event.        2. The density of charged ink particles concentrated at an ejection location increases with the strength of the applied electric field. This is because the electric field causes electrophoresis that overcomes the natural propensity for the ink particles to disperse evenly (neglecting gravity) and attain a homogeneous density. Opposing this concentration effect is diffusion and the Coulombic repulsion between the like-charged particles. In a steady-state, the system will reach a concentration equilibrium characterised by a density of charged particles that is dependent on the magnitude of the applied d.c. bias VB. When a voltage pulse is applied to the ejection location, the concentration of ink particles increases until the electrophoretic force on the particles overcomes the surface tension of the ink and ejection occurs. After the voltage pulse has finished, the concentration of the particles will relax towards the initial equilibrium state, driven by diffusion and Coulombic forces. If a voltage pulse is applied during the relaxation process then the ejection will start from a more favourable state of increased particle concentration and the subsequent print density will be stronger.        
These processes have characteristic time-constants that are of the order of one to a few hundred microseconds. In order for the printed image to be robust against the effects of start-up and stop delay compensation can be provided via the drive voltage waveforms. Such compensation is termed ‘history correction’.
Electrostatic inkjet printheads can be controlled using the duration and/or amplitude of electrical pulses to the printhead ejectors to modulate the ejection from the ejectors. Unlike piezo or thermal inkjet printheads, in which the size of droplet ejected is primarily a function of the physical dimensions of the pressure chamber and nozzle, the volume of ink ejected from an electrostatic printhead ejector can be controlled by the amplitude and/or the duration of the electric field acting on the ink in the ejector, which in turn is determined by the voltage waveform applied to the electrodes of the printhead. This enables predictable variations in the ejection performance of an ejector to be corrected by the drive waveform to the ejector.
The ways in which the drive pulse duration and amplitude can be controlled are shown schematically in FIGS. 4 & 5.
One solution to history effects is to vary the waveform of the voltage pulse as a function of the duration since the previous pulse. A way of implementing this is described in JP10258511 and is expanded upon in U.S. Pat. No. 7,172,267. These inventions describe a scheme where the duration of the pulse is increased as the duration since the previous ejection is increased. This solution works, but is difficult to implement in practice, as the controlling electronics and software must be able to cope with pulses of highly variable duration that extends to greater than the pixel period, necessitating a greater bit-depth of data specifying pulse length if resolution is not to be sacrificed, and consequently more complex electronics.